In recent years, a display device including a light emitting element has been developed. The display device including a light emitting element has the features such as fast response and wide viewing angle, in addition to the advantages of an existing liquid crystal display device, such as high image quality, thinness and lightweight. Therefore, it has been developed mainly for use as portable terminal. A light emitting element comprises between two electrodes a layer formed of various materials such as an organic material and an inorganic material.
A light emitting material has the feature that the luminance thereof degrades with time. Thus, in order to suppress the degradation of a light emitting element and improve the reliability, there is a method that applies a reverse bias voltage to the light emitting element (see Patent Document 1). Furthermore, there is a display device comprising in one pixel an EL driving TFT that is connected in series with a light emitting element and controls light emission of the light emitting element, a switching transistor (also called a writing transistor) for controlling a video signal input to the pixel, and an erasing TFT (also called a reset transistor) for controlling on/off of the EL driving TFT (see Patent Document 2).    [Patent Document 1] Japanese Patent Application No. 2001-117534    [Patent Document 2] Japanese Patent Application No. 2001-343933